The circuits used to perform the amplification functions include components based on monolithic microwave integrated circuits (MMICs). Such MMIC components include integrated circuits connected together via transmission lines. The architecture, the lengths, and the layout of the transmission lines inside such an MMIC component are of types such as to constitute a microwave amplification circuit that is intrinsically non-oscillatory. In practice, a microwave amplification assembly includes an insulating plate (e.g. made of ceramic, of polytetrafluoroethylene-glass, or of epoxy resin) in which recesses are machined for receiving the MMIC components. The insulating plate carries conductive tracks for interconnecting the components. Each of the MMIC components has an input via which a signal to be amplified is injected, and an output via which the amplified signal is delivered. AN MMIC component is often in the shape of a rectangular block, the input being situated on one side of the MMIC component, and the output being situated on a side opposite from the input side.
The amplified signal radiates naturally in the space situated in the vicinity of the output of the component. Unfortunately, the radiation then propagates to the input. As a result, the signals produced at the output tend to be re-injected into the input. This phenomenon is a radio transmission phenomenon.
To limit the disturbing effect of such phenomena, MMIC components are provided with metal covers and with surrounding absorbent materials. Thus, absorbent materials are placed between the integrated circuits of the MMIC component and the cover. In addition, the MMIC component provided with its cover is itself placed in a housing having an input facing the input of the MMIC component and an output facing the output of the MMIC component. The space between a lid of the housing and the cover is also filled with absorbent materials so as to prevent recursive propagation of amplified waves. The housing is connected to other circuits via shielded link cables.
A drawback suffered by such absorbents is that they do not perform their function satisfactorily. Even if care is taken in choosing the absorbents and their layout, the amplification limits that can be reached with such an amplification circuit, in its housing, are about 40 dB. Beyond such limits, and for certain spectrum components of the frequency band, the MMIC component starts oscillating (the gain is very large), or the re-injection gives rise to considerable attenuation. In the working band, the gain actually achieved varies unpredictably well above or well below the amplification value assigned to it. Such amplifiers are therefore unusable; they produce too much distortion of the amplified signal.
To remedy that problem, e.g. to achieve amplification of 90 dB, provision is made to cascade a plurality of housings, e.g. three housings, containing respective amplification circuits. Naturally, that increases the price substantially threefold.
In addition, the absorbent materials suffer from the drawback of degassing. By sublimation, they diffuse particles which, in particular, penetrate into the conduction channels of the gallium arsenide (GaAs) transistors used in the integrated circuits. In addition, between the cover and the lid, the damping power of the absorbent materials is, in practice, limited to 50 dB at best.